A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. comprising part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
United States patent application publication US 2004-0105082 describes the use of a plasma source to produce radiation for use to form a patterned beam to transfer a pattern to a substrate. In an embodiment, it is described how the Z-pinch effect of plasma affects the production of radiation. Briefly, Z-pinch involves the interaction of plasma current and the azimuthal magnetic field induced by that plasma current (i.e., the magnetic field directed tangentially to a circle in a plane perpendicular to the plasma current). The azimuthal magnetic field causes a force on the plasma carried discharge current. This force is directed to the central axis of the discharge. As a result, the plasma with current is compressed (pinched) and the intensity of radiation from the plasma is raised due to increased Joule heating of the pinched area. This radiation may be suitable for EUV lithography.
In an EUV lithographic apparatus, reflective elements (e.g., mirrors shaped to have a focusing effect, etc.) are used to form a beam from the radiation emitted by the Z-pinch plasma current, to illuminate the patterning device with the beam and to project the patterned beam from the patterning device onto the substrate.
Apart the EUV radiation, the Z-pinch plasma also emits fast ions. Unfortunately, these fast ions may cause damage to the reflective elements in such a lithographic apparatus.